This invention relates to a method of rolling H-beams or wide flange beams by universal mills, which is advantageously used in case of continuously producing H-beams while keeping a constant web width in spite of wear of the rolls used in rolling, or producing various H-beams different in size with the same rolling installation.
In general, H-beams are produced by hot rolling steel blanks 5, 6 or 7 having various cross-sections as shown in FIGS. 2a to 2c in a line including a breakdown mill 1, a universal roughing mill 2, an edger mill 3 and a universal finishing mill 4 which are arranged progressively downstream of the flowing of the steel blanks as shown in FIGS. 1a and 1b.
The blanks (slab 5, rectangular billet 6 and H-beam billet 7) shown in FIGS. 2a to 2c are first roughly rolled to predetermined shapes in the breakdown mill 1. The breakdown mill 1 used in this case is usually composed of a pair of upper and lower caliber rolls having open passes 8 or closed passes 9 as shown in FIGS. 3a and 3b. In rolling in the breakdown mill 1, the steel blanks are rolled successively through a plurality of passes of the caliber rolls in plural passes to be rolled into shapes suitable for later intermediate rolling processes
The steel blanks thus rolled are intermediately rolled in at least one universal roughing mill 10 having rolls of shapes shown in FIG. 4a and at least one edger mill 11 having rolls of shapes shown in FIG. 4b in one pass or plural passes. The steel blanks are then rolled in a universal finishing mill 12 having rolls of shapes shown in FIG. 4c usually in one pass to H-beam steel products. Therefore, sizes of rolls of the finishing universal mill 12 and the rolling mills upstream thereof are determined depending upon the size of the products. The rolls are so designed that distance (a) in FIG. 3a and distances (b), (c) and (d) in FIGS. 4a, 4b and 4c are substantially equal to each other.
In rolling for producing H-beams, variation in shape of blanks particularly after breakdown rolling is limited as described above. In the case that a particular series of H-beams (for example, H 600.times.300) are pro practice to use horizontal rolls having particular widths suitable for the H-beams.
The H-beams rolled by such horizontal rolls having the particular widths (for example, the width (d) in FIG. 4c) have substantially constant inner web widths. On the other hand, roll gaps between the horizontal rolls and between vertical rolls must be changed in order to roll one series of section steels of several kinds having different thicknesses by the use of the same rolls without exchanging them. In this case, the difference between the maximum and minimum thicknesses of flanges of the rolled H-beams becomes, for example, as much as approximately 16 mm. As the outer web width is an inner web width plus thicknesses of two flanges, the outer web width varies within 32 mm which is twice 16 mm.
It is unavoidable to produce a series of H-beams including those of various outer web widths in the rolling methods of the prior art described above. If such H-beams are used as building or construction beams, there are the following problems.
In the event that building or construction beams are made by joining a series of H-beams of several sizes, when the H-beams including those of various outer web widths are arranged so that outer surfaces of one flanges of the respective H-beams are in a plane, outer surfaces of the other flanges of the H beams are located unevenly with difference in height of twice the difference in thickness of the flanges. Such an unevenness provides a great problem to be solved in constructing the building or construction beams.
In designing structures of buildings, dimensions are usually determined successively from outside to inside of the structure. Therefore, such H-beams whose inner web widths are substantially constant but outer web widths are different depending upon thicknesses of flanges of the H-beams encounter a great problem in cases where adjustment of dimensions relative to each other at joined portions of the H-beams must be severely accurate.
In order to avoid the above disadvantages of the H-beams produced by rolling, H-beams made of steel plates by welding have been used particularly for buildings, which are welded to form H-beams having constant outer web widths, even if thicknesses of their flanges are not uniform. However, such welded H-beams are disadvantageous because of high manufacturing cost.
In order to solve the problem of the unevenness of outer web widths of H-beams, a method of adjusting the outer web widths was disclosed in Japanese Patent Application Laid-open No 59-202,101, in which rough rolled billets before finish rolling are rolled to widen their webs by a particular rolling mills having rollers inclined relative to rolling directions and supported by cantilevers (FIGS. 5a and 5b). In this method, however, the particular rolling mill for widening the webs is needed to increase the installation cost. Moreover, there is a risk of webs to be broken when billets have thin webs.
In order to overcome these problems, a method of reducing outer web widths by vertical rolls of a universal mill was proposed, whose horizontal rolls have widths narrower than inner web widths of H-beams to be produced, as disclosed in Japanese Patent Application Laid-open No. 2-84,203 (FIG. 6). Moreover, a method of reducing outer web widths by a universal mill was proposed, whose vertical rolls are set so as to permit the distance therebetween or roll gap to be less than the width of horizontal rolls plus sum of thicknesses of both flanges and are shifted onto the downstream side relative to the horizontal rolls so as to avoid any interference of the vertical and horizontal rolls with each other, as disclosed in Japanese Patent Application Laid-open Nos. 2-147,102 and 2-147,112 (FIGS. 7a and 7b). In this case, existing rolls used until now are applicable for carrying out these methods and these rolling mills are not needed to be particularly modified. Therefore, these methods can be readily effected.
With these methods, however, defects of H-beams are often caused such as overlapping at rounded portions, buckling of webs or shifting of webs from center positions, when rolling reduction of the inner web widths is relatively large.
In view of these disadvantages, the inventors of this invention of the present application propose a method of reducing or adjusting inner web widths of H-beams by finish rolling as disclosed in Japanese Patent Application Laid-open No. 2-80,102. In this method, rough rolled billets after breakdown and intermediate rolling are rolled by finish rolling with a finish rolling mill whose horizontal rolls (adjustable width rolls) are set to have roll widths less than those in the rough rolling. As a result, the billets are subjected to the finish rolling to reduce web widths and thicknesses of flanges and to correct inclination of the flanges. In this manner, the web widths are freely adjusted or reduced (FIGS. 8a and 8b). According to this method, H-beams having constant outer web widths can be effectively produced, even if rolling is applied to billets to modify thicknesses of their flanges. Even in this method, however, the reduction of the outer web widths is limited as explained hereinafter. Therefore, a rolling system has been expected which is able to realize larger reduction of web widths.
In reducing the web widths by setting the roll widths of the horizontal rolls of the finishing mill less than the inner web width of the billet subjected to the rough rolling, the contacting state between the rolls of the rolling mill and the billet is as shown in FIG. 9.
Referring to FIG. 9, as the inner web width Bw.sub.0 is reduced by the vertical rolls V, they contact the billet h prior to contacting of the horizontal rolls H with a normal rolling reduction and normal roll diameters so that the web width of the billet h is reduced until end surfaces of the horizontal rolls H contact the billet h. The reduction of the inner web width Bw.sub.0 is effected mainly at zones located slightly upstream of zones k where the horizontal rolls contact the web ha of the billet h. On the other hand, before the contact of the horizontal rolls with the web ha of the billet h, roll gaps between the upper and lower horizontal rolls H are more than the thickness of the web ha as shown in FIG. 10a. Therefore, buckling or torsion of the web ha may occur as the case may be as shown in FIGS. 10b and 10c. As the web ha of the billet rolled by rough rolling is rolled to reduce its thickness by the horizontal rolls H, the billet h rolled by finish rolling will be shaped substantially determined by the roll gaps between the upper and lower horizontal rolls H even if buckling occurs before the reduction by the horizontal rolls H. However, upon amending the buckling of the web ha by the reduction caused by the horizontal rolls H, contacting pressure between the web ha and the horizontal rolls H becomes locally higher to cause defects such as flaws or cracks in surfaces of the web ha.
Moreover, the torsion of the web ha as shown in FIG. 10a or 10c permits the billet to pass through the finishing mill in a condition of longitudinal center lines of the flanges hb shifted from the roll gaps between the horizontal rolls H. Consequently, the web ha of finished product is often shifted relative to the flanges hb in opposite directions or one direction as shown in FIG. 11a or 11b.
The thinner and wider the webs of billets before finish rolling, these problems are particularly acute. Moreover, when the reduction or adjustment of web widths is larger, the possibility of occurrence of such defects increases.
The thicknesses of webs of billets before finish rolling are determined by appropriate rolling reduction in universal rolling. On the other hand, the inner web widths of billets before finish rolling are substantially equal to inner web widths of billets having the thinnest flange thicknesses in one rolling operation. Therefore, in order to prevent the defects in finish rolling described above, it is necessary to provide a limitation of rolling reduction in one pass according to thicknesses and inner widths of webs. If a required rolling reduction exceeds this limitation, the rolling is required to be divided into two or more passes.
Referring back to the prior art methods described above, the method previously proposed (the Japanese Patent Application Laid-open No. 2-80,102) by the inventors of the present invention is fundamentally different from the other methods (Japanese Patent Application Laid-open Nos. 2-84,203, 2-147,102 and 2-117,112) in the feature of rolling to reduce web widths and substantially at the same time to reduce flange thicknesses. According to the method proposed by the inventors of the present invention, it is possible to make larger the rolling reduction of the flanges than that of the webs in universal finishing process so that surfaces of the webs being rolled are subjected to tensile stresses in rolling directions caused by elongations of the flanges. As a result, it is possible to mitigate the limitation of reduction of webs to a remarkable extent for preventing the buckling of the webs caused by the compression in directions perpendicular to the rolling directions. According to this method, therefore, the rolling reduction or adjustment of web widths can be increased more than three times in comparison with those in the other prior art methods, although the adjustable width horizontal rolls are needed.
However, if a required adjustment of web widths exceeds a limitation, it is necessary for reducing web widths to divide the rolling into plural passes more than two. In the method previously disclosed by the inventors of the present invention in the Japanese Patent Application Laid-open No. 2-80,102, the rolling is carried out to fulfill the condition .DELTA.Bwmax=80 Tw.sup.2 /Bw, where .DELTA.Bwmax (mm) is the limit value of rolling reduction of inner web width, Tw.sup.2 (mm) is web thickness before being rolled, and Bw (mm) is inner web width. In other words, if a rolling reduction .DELTA.Bw of inner web widths exceeds the .DELTA.Bwmax calculated from the above equation, the rolling is divided into more than two passes to limit the rolling reduction per one pass.
However, when the rolling is effected in more than two passes in the finish rolling, the temperature of the steel to be rolled is likely to lower. Such a temperature lowering of the steel often causes not only defects of products in shape such as waved webs and deterioration of product quality but also lowering of production efficiency. It is, therefore, preferable to perform the rolling in one pass. Moreover, it becomes clear that more severe reduction limitations of web widths are often needed in actual rolling operations.
In rolling causing rolling reduction of web widths, buckling and detrimental deformation in section of products can be prevented by arranging restraining means such as web guides on the entrance side of a rolling mill. However, such means do not serve to enlarge the rolling reduction or adjustment per one pass.
In this connection, the rolling reduction or adjustment of the inner web widths can be effected partially in rough rolling processes. However, as large rolling reductions with adjustable width rolls tend to cause stepped surfaces of products, thicknesses of webs cannot be considerably reduced by the rolling. Consequently, an exclusive pass is needed for reduction of web widths so that the number of passes increases and hence to encounter the difficulties described above.